1. Field of the Invention
This invention relates to a spot weld inspecting device for inspecting the size of a nugget in a spot weld by the use of an ultrasonic wave.
2. Description of the Prior Art
Acceptability of a spot weld depends greatly on whether or not the nugget is formed as predetermined. In the case of a good weld, the size of the nugget is slightly larger than or close to the diameter of an electrode tip used in the welding. In the case of a poor weld, the size of the nugget is much smaller than the diameter of the electrode tip, and the weld includes portions not welded. In order to inspect whether or not the nugget is formed to a size which is required for sufficient strength, a spot weld inspecting device using an ultrasonic wave is employed.
A conventional spot weld inspecting device, as shown in FIG. 1 comprises a probe 1, and a display unit 2 such as an oscilloscope. The probe 1 is disposed in contact with a material 3 to be inspected in which plates 31 and 32 are welded together. The probe transmits an ultrasonic wave thereto and receives a wave reflected therefrom. The display unit 2 operates to receive an electrical signal from the probe 1 and to thereby display the reflected wave 21.
However, the conventional probe 1 emits the ultrasonic wave with the entire end face 11 thereof (having substantially the same diameter as that of the welding tip) in contact with the inspection material. Therefore, in the case of a small nugget 4, both the information (reflected wave) from a portion where the nugget 4 is formed and the information from a portion not welded are unseparated when they are received by the probe 1. Accordingly, it has been difficult to determine whether or not the size of the nugget is as large as, or is larger than, the value predetermined to obtain sufficient strength.
In the conventional device, the probe 1 emits an ultrasonic wave pulse in the direction of the inspection material which has been spot-welded through one plate 31 thereof and receives the reflected wave from the inspection material. In the case where the size of the nugget 4 is smaller than the end face 11 of the probe, the portion of the incident wave in the area where the nugget has been formed, passes through the nugget and produces multiple reflection waves 51 between the outer surface of the other plate 32 and the acoustic wave incident surface of the plate 31, while in the area where the nugget has not been formed, multiple reflection waves 52 are produced between the surface of the plate 31, which confronts the plate 32, and the incident surface. These reflected waves 51 and 52 are received by the probe 1 and are displayed on the oscilloscope. Therefore, the reflected waves 52 produced in the portion welded unsatisfactorily appear between the pulses of the reflected waves 51 produced in the portion where the nugget has been formed and welding has been satisfactorily done.
In this case, in order to determine the size of the nugget from the reflected waves 51 and 52, it is necessary to measure the height of the reflected waves 52 in the unsatisfactorily welded portion (the strength of the reflected waves) and to utilize the analytical curve obtained from the relationships between reflected wave height and nugget diameter. However, this method is rather intricate and is low in accuracy.
Thus, the conventional device is disadvantageous in that it cannot be readily utilized to determine whether the weld is satisfactory or not.
In order to solve these problems, the inventors have developed a spot weld inspecting device in which an ultrasonic wave propagating wave guide is arranged on the tip of a probe (Japanese Patent Application No. 105087/1970, published as Japanese Patent Publication No. 15677/1975 and patented as Japanese Patent No. 798206). The wave guide, as shown in FIG. 17, comprises a cylindrical structure 62A serving as an ultrasonic wave propagating section and an ultrasonic wave absorbing material 63A filling a cavity 64A formed in the cylindrical structure 62A. The cylindrical structure 62A is provided with an annular contact surface 65A at one end which may be brought into contact with the inspection material. In FIG. 17, reference character 61A designates a recess into which the probe 1 is inserted, and reference character 612A designates a probe connecting surface. In the wave guide, the outside diameter of the contact surface 65A is substantially equal to the outside diameter of an electrode tip for welding and the inside diameter thereof corresponds to the size of a nugget of the minimum size required for welding.
As the wave guide is thus constructed, the information on the central portion of a weld is not received by the probe 1 but the information on the peripheral portion thereof is received by the probe. Therefore, the problems accompanying the above-described conventional device can be solved. More specifically, in inspection, when the size of a nugget is smaller than the size required for welding, the size of the nugget is smaller than the inside diameter of the annular contact surface 65A of the wave guide 6A, and therefore only the information on the not-welded portion is supplied to the wave guide. This information (multiple reflection) is supplied through the probe 1 to the display unit 2, whereby the poor weld can be easily determined. On the other hand, when the size of a nugget is larger than the minimum size required for welding, the size of the nugget is also larger than the inside diameter of the annular contact surface 65A, and therefore the information (multiple reflection) on the welded portion is also supplied to the wave guide 6A, whereby the good weld is determined by a method similar to that in the above-described case.
Thus, the difficulties accompanying the conventional device are eliminated by the provision of a device using the wave guide earlier developed by the inventors.
However, as the distance between the annular contact surface 65A and the probe connecting surface 612A is relatively long, i.e. about 10 mm, in the wave guide (hereinafter referred to as a conventional wave guide, when applicable) in the device developed by the inventors, the following problem is involved when thin plates 1.4 mm or less in thickness are welded.
In the case where inspection is performed with the conventional wave guide, if a probe capable of emitting a strong ultrasonic wave pulse at a relatively low frequency (lower than 5 MHz) is used, the size of a nugget in a spot weld of a plate whose thickness is more than 1.6 mm can be inspected with high accuracy; however, there is a fear that it would be impossible to inspect a spot weld of plates whose thickness is less than 1.4 mm because the individual pulses of multiple reflection in the spot weld could not be separated from one another.
Therefore, the pulse resolution may be improved by using a probe capable of emitting an ultrasonic wave at a high frequency (higher than 10 MHz) or by using a high resolution probe capable of producing pulses in the form of a shock wave, which are short in pulse width. However, since these probes cannot product strong ultrasonic wave pulses, they cannot receive the multiple reflection waves in spot welds with sufficiently high sensitivity. Thus, these probes cannot be utilized for inspecting spot welds of thin plates as described above.
The strength of the ultrasonic wave pulse incident to a spot weld may be increased by decreasing the length of the ultrasonic wave propagating section in the wave guide. However, if the length of the ultrasonic wave propagating section is merely reduced, after the first ultrasonic pulse emitted by the probe has propagated through the wave guide and entered the weld, a part of the energy of the ultrasonic pulse causes reflections to occur repeatedly in the ultrasonic wave propagating section between the annular contact surface 65A and the internal flat surface 612A of the wave guide to thereby produce second, third, fourth, etc., unwanted reflected waves which enter the weld. Accordingly, each of the number of unwanted incident waves causes multiple reflection waves in the weld, which are superposed on the necessary multiple reflection waves in the weld caused by the first incident pulse. Thus, it is impossible to clearly receive the multiple reflection wave in the spot weld merely by reducing the length of the ultrasonic wave propagating section.
However, this problem is not created in the case where, as in the conventional wave guide, the length of the ultrasonic wave propagating section is relatively long, as 10 mm, because the reflected wave pulses of the above-identified type are damped while passing through the long ultrasonic wave propagating section. However, as was described above, with such a long ultrasonic wave propagating section, it is impossible to inspect the spot weld of thin plates.